1. Field of the Invention
The present invention relates generally to the field of diagnostic tests and, more particularly, to test devices and elements useful in the qualitative and quantitative determination of an analyte using a composition which includes a tetraalkylbenzidine indicator.
2. Brief Description of the Prior Art
Test devices in the form of test strips and similar solid state analytical elements have become commonplace in the analysis of various types of samples, particularly biological fluids. Test strips designed for detecting clinically significant substances in biological fluids, such as serum and urine, have been advantageous in the diagnosis of disease.
Test strips of various types have been known and used for many years in a wide variety of fields, from the most familiar pH test paper devices to in vitro diagnostic devices for the detection of various urine and blood components such as glucose, protein, occult blood and so forth (e.g., as described in U.S. Pat. Nos. 3,164,534; 3,485,587; and 3,012,976).
The chromogenic indicator o-tolidine has been used for some time in test compositions, but provides results which are subject to reduction of the oxidized indicator by interfering substances, such as ascorbic acid. Further, the safety of o-tolidine has been questioned. British published patent specifications 1,464,359 and 1,464,360 disclose the use of 3,3',5,5'-tetramethylbenzidine and similar compounds and their use in the detection and determination of hydrogen peroxide or of constituents which react to form peroxides.
U.S. Pat. No. 4,273,868, commonly assigned herewith, discloses a composition, a test device, method of making the test device and process for determining glucose in a sample. The test composition comprises glucose oxidase, a peroxidatively active substance, such as peroxidase, a stabilizing agent and a 3,3',5,5'-tetraalkylbenzidine indicator in an amount sufficient rapidly to produce, upon contact of the test means with a predetermined amount of a glucose-containing sample, a stable colored reaction product believed to comprise reduced and oxidized forms of said indicator in stable equilibrium. Preferably, the 3,3',5,5'-tetramethylbenzidine is present in a concentration of at least about 2.6 millimoles per thousand International Units of glucose oxidase activity. One of the disclosed stabilizing agents is an interpolymer of methylvinyl ether and maleic anhydride, marketed commercially as Gantrez AN-139 by GAF Corporation. The test devices are prepared by a two-dip impregnation process where the 3,3',5,5'-tetraalkylbenzidine is impregnated in the second dip using a solution thereof prepared in an organic solvent.